nextcloud: Support remote database server

The _nextcloud_ role originally handled setting up the PostgreSQL
database and assumed that it was running on the same server as Nextcloud
itself.  I have factored out those tasks into their own role,
_nextcloud-db_, which can be applied to a separate host.

I have also introduced some new variables (`nextcloud_db_host`,
`nextcloud_db_name`, `nextcloud_db_user`, and `nextcloud_db_password`),
which can be used to specify how to connect to the database, if it is
hosted remotely.  Since these variables are used by both the _nextcloud_
and _nextcloud-db_ roles, they are actually defined in a separate role,
_nextcloud-base_, upon which both depend.

roles/nextcloud-db/tasks/main.yml

@@ -0,0 +1,19 @@
+- name: ensure nextcloud database user exists
+  become: true
+  become_user: postgres
+  postgresql_user:
+    name: '{{ nextcloud_db_user }}'
+    password: '{{ nextcloud_db_password|d(omit) }}'
+    state: present
+  tags:
+  - postgresql-user
+
+- name: ensure nextcloud database exists
+  become: true
+  become_user: postgres
+  postgresql_db:
+    name: nextcloud
+    owner: nextcloud
+    state: present
+  tags:
+  - postgresql-db